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Experimental Studies of W/Z+ Jets and W/Z+ Heavy Flavor Jets at the Tevatron

This presentation discusses the importance of studying W/Z+ jet production and recent progress at the Tevatron. It also highlights the need for experimental verification of recent simulations and the shared signature with other searches at the Tevatron and LHC.

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Experimental Studies of W/Z+ Jets and W/Z+ Heavy Flavor Jets at the Tevatron

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  1. Experimental Studies of W/Z + Jets and W/Z + Heavy Flavor Jets at the Tevatron Christopher Neu on behalf of the CDF and D Collaborations HCP200819th Hadron Collider Physics Symposium 2008 27 May 2008 Galena, IL Outline: • Importance of W/Z + jets • Recent Tevatron progress • Summary and future

  2. Importance of W/Z + Jet Physics Why study W/Z +jet production? q • Important tests of Quantum Chromodynamics (QCD) • Recent LO and NLO simulations need experimental verification • Signature shared with top production, Higgs, other searches at Tevatron, LHC q q(‘) q’ q’ V q q q q V q(’) V=W or Z NB: New DØ results coming this summer! Christopher Neu

  3. The CDF and D Experiments Common features: Christopher Neu Charged particle tracking in magnetic field Electromagnetic and hadroniccalorimetry Muon detection Luminosity monitoring Three level event trigger

  4. W + Inclusive Jets W + ≥ 2 jets W + ≥ 1 jet W+jets signal Fake W bkgd Real W bkgd W+jets signal Fake W bkgd Real W bkgd (tt,WZ,WW..) W + ≥ 3 jets S/B ~ 10/1 W+jets signal Fake W bkgd Real W bkgd W + ≥ 4 jets W+jets signal Fake W bkgd Real W bkgd S/B ~ 1.2/1 • W selection: seek W e  • e: ET > 20 GeV, || < 1.1 • : missing transverse energy MET> 30 GeV • MT (W) > 20 GeV/c2 • Jet definition: Cone algorithm, R= 0.4 • Corrected ET > 20 GeV, || < 2.0 Christopher Neu

  5. W + Inclusive Jets PRD 77, 011108(R) “MCFM” : MCFM (NLO) + no shower “MLM” : ALPGEN (LO) + Herwig (shower) + MLM matching “SMPR” : MadGraph (LO) + Pythia (shower) + CKKW matching Total cross section for jet multiplicity, n: “MCFM”: Monte Carlo for Femtobarn Processes “MLM”: M. Mangano “SMPR”: S. Mrenna & P. Richardson “CKKW” : Catani, Krauss, Kuhn, Webber Acronym key: NLO prediction more accurate than LO! …and relative rates from bin-to-bin consistent with data. Christopher Neu

  6. W + Inclusive Jets PRD 77, 011108(R) “MCFM” : MCFM (NLO) + no shower “MLM” : ALPGEN (LO) + Herwig (shower) + MLM matching Can examine differential cross sections for nth jet within each multiplicity bin…. “SMPR” : MadGraph (LO) + Pythia (shower) + CKKW matching Total cross section for jet multiplicity, n: “MCFM”: Monte Carlo for Femtobarn Processes “MLM”: M. Mangano “SMPR”: S. Mrenna & P. Richardson “CKKW” : Catani, Krauss, Kuhn, Webber Acronym key: NLO prediction more accurate than LO! …and relative rates from bin-to-bin consistent with data. Christopher Neu

  7. W + Inclusive Jets W+2 jets PRD 77, 011108(R) “MCFM” : MCFM (NLO) + no shower “MLM” : ALPGEN (LO) + Herwig (shower) + MLM matching “SMPR” : MadGraph (LO) + Pythia (shower) + CKKW matching At LO, MadGraph+Pythia+CKKW provides better performance. Christopher Neu • LO calculation procedure: Generate ppW+Npartons at tree level, ignore loop corrections, employ parton shower • Ambiguities arise: • Possibility for double counting if Nparton  Njet • SMPR and MLM refer to algorithms for avoiding/removing overlaps

  8. W + Inclusive Jets W+2 jets PRD 77, 011108(R) “MCFM” : MCFM (NLO) + no shower “MLM” : ALPGEN (LO) + Herwig (shower) + MLM matching “SMPR” : MadGraph (LO) + Pythia (shower) + CKKW matching But why? Is it the matrix element? Shower? Matching? Work is ongoing. At LO, MadGraph+Pythia+CKKW provides better performance. Christopher Neu • LO calculation procedure: Generate ppW+Npartons at tree level, ignore loop corrections, employ parton shower • Ambiguities arise: • Possibility for double counting if Nparton  Njet • SMPR and MLM refer to algorithms for avoiding/removing overlaps

  9. Z /* + Inclusive Jets Phys. Rev. Lett. 100, 102001 (2008) Can’t see the NLO prediction points - close overlap with data! NLO prediction once again more accurate than LO! Christopher Neu • Validity of NLO predictions borne out in Z /*+jets? • Z /* selection: seek Z /*e+e- • Two ET > 25 GeV electrons • 66 < Mee < 116 GeV/c2 • Jet definition: • Corrected pT > 30, |y| < 2.1 • Cone algorithm , R=0.7 • Major backgrounds: S/B ~ 7/1 • QCD multijets • W + jets • ttbar, diboson • Z+, Z

  10. Phys. Rev. Lett. 100, 102001 (2008) Z /* + Inclusive Jets Christopher Neu • Differential cross section: • NLO was good in W+jets, true here too? NLO prediction reliable – as in W+jets • Analysis would benefit from increased statistics to further populate the Z+2-jets sample • NLO for Z+3-jets would be valuable as well.

  11. Z /* + Inclusive Jets Pythia Sherpa Christopher Neu • DZ /*(ee)+jets analysis: 950/pb • Purpose here: compare Pythia(ppW+1p+ internal PS)and Sherpa (ppW+Np + internal PS + CKKW matching) event generators • Test of different prediction techniques • Some confidence in CKKW from CDF W+jets LO studies…true here as well?

  12. Z /* + Inclusive Jets Pythia Sherpa Sherpa + CKKW represents data better than Pythia - pT of jet 1,2,3 - Z pT, Jet multiplicity - (jet, jet), (jet, jet) Not unexpected given the nature of Pythia’s calculation. Christopher Neu

  13. top pair production s-channel single top W/Z H production Summary so far… ℓ / ℓ,  / ℓ,  W/Z W/Z • W/Z+1,2 jet NLO predictions from MCFM look reliable • NLO predictions not yet in hand for W/Z+3 jet • Technique of calculating/generating ppW+N+ parton shower + matching scheme (ala ALPGEN, MadGraph, Sherpa) superior to Pythia+PS alone • Differences among available tools still need to be understood • W/Z + heavy flavor (b,c) jets also important • background to top, Higgs, others • W+c production has unique features Christopher Neu

  14. W + Single c Production c c W- s(90%) or d(10%) Soft Muon Identification Parameterization for “mistags”: - decays in flight - hadronic punch-through c or c μ+ or μ- Christopher Neu • Importance of W± +single c: • Insight on PDF for s at rather large Q2 • Insight on |Vcs| • Part of W+jets bkgd to top, Higgs searches • Event selection similar to W+jets: • Here use W→e/µ νfor W selection • Exploit W± +single c feature: • charm hadron semileptonic daughter and W have opposite charge • Major opposite-sign (OS) backgrounds: • Drell Yan μ+μ- • Fake W • Wq • Insensitive to W+bb, W+cc, (OS/SS random)

  15. W + Single c Production • Result: for pTc > 20, |ηc|<1.5xBR = 9.8  2.8 (stat)+1.4 -1.6(syst)  0.6 (lum)pb • Prediction: NLO from MCFM • xBR = 11.0 +1.4 -3.0pb Good agreement! Phys. Rev. Lett. 100, 091803 (2008) Christopher Neu

  16. W + Single c Production Submitted to Phys. Lett. B - arXiv:/0803.2259 [hep-ex] Statistics limited measurement Systematics dominated by JES. LO prediction reasonably good. Christopher Neu • Similar analysis completed at D: 1/fb • Measures the ratio which allows for cancellation of many systematic errors • Result: which can be compared to the LO prediction: 0.040  0.003 (PDF)

  17. Vertex Tagging:b’s and Non-b’s Tag efficiency for b jets Tagging of real b jet: long lifetime+ large boost = secondary vertex Displaced tracks Secondary vertex Fractional  L2d > 0 Primary vertex Prompt tracks Jet ET (GeV) Displaced tracks Tag efficiency for u/d/s jets Spurious tagging of light flavor jet: “mistag” Fractional  Secondary vertex Primary vertex Prompt tracks L2d<0 Jet ET (GeV) Christopher Neu

  18. Vertex Mass Shapes = u/d/s/g W + b-Jets From simulation • Generally, • MB-hadrons≳ MC-hadrons≳ MLF-hadrons • so • Mb vert≳ Mc vert≳ MLFvert • Goals: • Measure W+b-jet production cross section • Use measurement to improve background estimate for Higgs search • W and jets selection here similar to W + inclusive jets analysis • key difference: 1 or 2 jets only • Here we need to identify jets that are likelyb’s (via high purity tagging) and determine how many are reallyb’s via vertex mass: • invariant mass of charged particle tracks in secondary vertex Christopher Neu

  19. W + b-Jets High purity b-tagging at work! New result - x3.5 mismatch ~1000 tagged jets among which ~700 are consistent with coming from a b quark NB: This cross section is for b jets from W+b-jet production in events with a high pT central lepton, high pT neutrino and 1 or 2 total jets. Publication in preparation. • Largest backgrounds: S/B ~ 3/1 • ttbar (40% of total bkgd) • single top (30%) • Fake W (15%) • WZ (5%) • Total contribution: ~180 tagged b jets • Result: measureσb-jets(W+b-jets) x BR(W→lν) xBR = 2.74  0.27 (stat)  0.42 (syst) pb • Prediction: xBR = 0.78 pb (default ALPGEN) Christopher Neu

  20. W + b-Jets High purity b-tagging at work! New result - x3.5 mismatch Other predictions? Work is ongoing. ~1000 tagged jets among which ~700 are consistent with coming from a b quark NB: This cross section is for b jets from W+b-jet production in events with a high pT central lepton, high pT neutrino and 1 or 2 total jets. Publication in preparation. • Largest backgrounds: S/B ~ 3/1 • ttbar (40% of total bkgd) • single top (30%) • Fake W (15%) • WZ (5%) • Total contribution: ~180 tagged b jets • Result: measureσb-jets(W+b-jets) x BR(W→lν) xBR = 2.74  0.27 (stat)  0.42 (syst) pb • Prediction: xBR = 0.78 pb (default ALPGEN) Christopher Neu

  21. Z + b-Jets • Differential cross sections with comparisons to LO, NLO predictions • Dividing by (Z) puts LO, NLO on equal footing • Pythia does a good job at low jet ET Christopher Neu • Similar CDF analysis for Z+b-jets: 2/fb • Utilize Zeeand  • Similar jet definition • Corrected ET > 20 GeV, || < 1.5 • Cone algorithm with R=0.7 • Secondary vertex tags

  22. Z + b-Jets • ALPGEN (LO) and MCFM (NLO) undershoot data in several bins • Pythia on target in some regimes – despite LO predictions being low in other analyses (eg, Z+jets). Publication in preparation. Christopher Neu

  23. W/Z + b-Jets: Summary Raw NLO predictions corrected for underlying event and hadronization effects. • More studies for W+b-jets are forthcoming • Need to understand NLO predictions • In Z+b-jets it is strange that the NLO prediction undershoots data • Borne out in W+b-jets? Christopher Neu

  24. Conclusions • W/Z + jets physics plays an important role in current collider physics programs • Current NLO predictions for W/Z + look to be accurate, higher multiplicities desirable • W/Z+b-jets studies have indicated deficiencies in both LO and NLO predictions; more study and more data is needed • W+singlec studies indicate reasonable agreement with NLO, LO predictions Christopher Neu

  25. Backup Slides Christopher Neu

  26. W + Inclusive Jets MCFM : MCFM (NLO) MLM : ALPGEN (LO) + Herwig (shower) + MLM matching SMPR : MadGraph (LO) + Pythia (shower) + CKKW matching Christopher Neu

  27. W + Inclusive Jets: Definition of Terms MCFM : MCFM (NLO) MLM : ALPGEN (LO) + Herwig (shower) + MLM matching SMPR : MadGraph (LO) + Pythia (shower) + CKKW matching Christopher Neu • MCFM: Monte Carlo for Femtobarn Processes • NLO predictions for cross sections and kinematics • MLM: Michelangelo Mangano, author of ALPGEN • ALPGEN, MadGraph: matrix element generators • Generate fixed order processes (eg., W+0,1,2,3 partons for W+jets) • Shower the N-parton final state to get N-jets (eg. Pythia or Herwig) • Gather all the fixed order samples (eg., W+N-p for W+jets) • Remove double-counting via matching algorithm • MLM matching: • Allow event iffNjets = Npartons (exclusive) or Njets ≥ Npartons (inclusive) • CKKW matching: • Assign each event weights from αs nodes, legs • Veto event if event weight is below some cut • Use shower to add legs only up to some cutoff • SMPR: variant of CKKW, named after S Mrenna and P Richardson

  28. Identifying b Jets • Bhadron lifetime: ~1.5 ps • Large boost (v ~ 0.95c) means the B lifetime is long in the lab frame • B travels macroscopic distance before decaying which we can detect • Exploit the long lifetime - • Reconstruct charged particle tracks • See if they intersect at a common point • Require the common point be significantly displaced from the primary p-p collision point Long-lifetime yields secondary decay vertex “b-tagging” Displaced tracks Secondary vertex d0 L2d Prompt tracks Primary vertex, aka p/pbar collision point Christopher Neu

  29. W + b-Jets Christopher Neu

  30. W + Single c Production µ pT relative to jet axis Signed µ track impact parameter significance. Christopher Neu

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